Method of Operating a Display Driver

ABSTRACT

A method of operating a display device comprising a drive circuit is disclosed. The drive circuit comprises a plurality of single grey-level channels, each comprising an input ( 412, 422 ), an output ( 418, 428 ) and a signal processor connected between the input and output. Each signal processor comprises a digital-to-analog converter ( 414, 424 ) and an operational amplifier ( 416, 426 ) having a voltage offset. The method comprises: converting a digital signal received at the input ( 412, 422 ) into an analog voltage ( 410, 420 ) at the output ( 418, 428 ) using each respective signal processor; switching between the analog voltage ( 410, 420 ) of each single grey-level channel using a switching circuit ( 430 ); receiving and analysing the analog voltages ( 410, 420 ) in a calibration subsystem ( 440 ), and individually compensating for the voltage offset of each op-amp ( 416, 426 ) based on the received analog voltage ( 410, 420 ) for that grey-level channel using the calibration subsystem ( 440 ).

FIELD

The present disclosure relates to a drive method for a display device.More particularly, the present disclosure relates to a drive method fora liquid crystal on silicon spatial light modulator. The presentdisclosure further relates to calibrating an operational amplifiercircuit for a plurality of grey-level voltages. Embodiments relate to amethod of operating a backplane for a liquid crystal on silicon spatiallight modulator. Embodiments relate to a method of calibrating a drivecircuit for a liquid crystal on silicon spatial light modulator and amethod of compensating for the random offset voltage of operationalamplifiers used in the drive circuit for a liquid crystal on siliconspatial light modulator.

BACKGROUND AND INTRODUCTION

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well-known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The hologram may be reconstructed by illuminationwith suitable light to form a two-dimensional (replay image) orthree-dimensional holographic reconstruction representative of theoriginal object.

Computer-generated holography may numerically simulate the interferenceprocess. A computer-generated hologram, “CGH”, may be calculated by atechnique based on a mathematical transformation such as a Fresnel orFourier transform. These types of holograms may be referred to asFresnel or Fourier holograms. A Fourier hologram may be considered aFourier domain representation of the object or a frequency domainrepresentation of the object. A CGH may also be calculated by coherentray tracing or a point cloud technique, for example.

A CGH may be encoded on a spatial light modulator, “SLM”, arranged tomodulate the amplitude and/or phase of incident light. Light modulationmay be achieved using electrically-addressable liquid crystals,optically-addressable liquid crystals or micro-mirrors, for example.

The SLM may comprise a plurality of individually-addressable pixelswhich may also be referred to as cells or elements. The light modulationscheme may be binary, multilevel or continuous. Alternatively, thedevice may be continuous (i.e. is not comprised of pixels) and lightmodulation may therefore be continuous across the device. The SLM may bereflective meaning that modulated light is output from the SLM inreflection. The SLM may equally be transmissive meaning that modulatedlight is output from the SLM is transmission.

A holographic projector may be provided using the described technology.Such projectors have found application in video projectors, head-updisplays, “HUD”, and head-mounted displays, “HMD”, including near-eyedevices, for example.

There is disclosed herein an improved drive solution for a displaydevice such as a spatial light modulator used for projection.

SUMMARY

Aspects of an invention are defined in the appended independent claims.

The inventor has provided an improved drive method for a display devicesuch as a spatial light modulator. Embodiments refer to a LCOS displaydevice by way of example only. The present disclosure is applicable toany display device which requires a plurality of analog voltages toprovide a plurality of grey levels or phase-delay levels. Embodimentsrefer to a drive circuit which provides 128 grey levels by way ofexample only. The present disclosure is applicable to any number of greylevels.

The drive circuit of the present disclosure uses a plurality ofoperational amplifiers, “op-amps”. However, operational amplifiersexperience a random offset voltage which reduces the precision withwhich their output voltages can be set. The inventor herein discloses asystem and method which reduce the voltage error caused by theoperational amplifiers. The present disclosure describes a method ofindividually compensating for the voltage offset experienced by eachoperational amplifier of a grey-level channel for driving a displaydevice. In embodiments, the method compensates for the voltage offset ofeach individual operational amplifier by calibrating a parameter of thecorresponding operational amplifier to provide a desired or target greylevel voltage. In other embodiments, the method compensates for thevoltage offset of each individual operational amplifier by determiningan input that provides the desired or target grey level voltage based onthe output response of the operational amplifier.

The present disclosure relates to a device wherein there is provided adigital-to-analog converter, “DAC”/op-amp pair that forms a signalprocessor for each grey level voltage. Accordingly, references herein to“calibrating a parameter” or “determining an input” of an operationalamplifier of a grey-level channel are intended to encompass calibratinga parameter or determining an input of a signal processor comprising aDAC/op-amp pair, or an individual DAC or op-amp of a signal processor ofa grey-level channel.

Embodiments refer to displaying a hologram on the display device by wayof example only. The present disclosure is equally applicable todisplaying a regular image on the display device. The present disclosureis applicable to displaying any type of information on the displaydevice using a plurality of voltage levels.

Reference is made throughout to “levels” including grey levels,modulation levels and phase-delay levels. The term “levels” is used inthis disclosure to mean discrete values. That is, the parameterdescribed may only take a value equal to one of a plurality of discretevalues. In other words, the parameter is constrained to particularvalues. For example, it is understood in the display industry that eachpixel of a display may modulate the intensity of light and may beoperated at a plurality of grey levels such as 128 grey levels fromblack to white, or vice versa. Each pixel may be described as a lightmodulating element operable at a plurality of modulation levels. Thelight modulating element may be a phase-modulating element operable at aplurality of phase-delay levels such as 0, π/2, π, π/2 and 2π. Referenceis made throughout to “grey levels” for convenience and consistency ofdisclosure only. In embodiments related to phase-modulating pixels, theterm “grey levels” could be read as “phase-delay levels”. In otherwords, in these embodiments, each “grey” is a “phase delay”. Forexample, grey level number 1 may be a phase-delay of 0 and grey levelnumber 128 may be a phase-delay of 2π.

Accordingly, references herein to “grey-level channel” refer to a drivepath or drive circuit, comprising the above described signal processorcomprising a DAC/op-amp pair, which provides a voltage level for drivingthe pixels of the display at the corresponding discrete grey level.Thus, during a particular display time interval (e.g. frame orsub-frame), a grey-level channel provides a fixed voltage level that isused to drive all of the pixels of the display that are intended todisplay the corresponding grey level.

The term “hologram” is used to refer to the recording which containsamplitude and/or phase information about the object. The term“holographic reconstruction” is used to refer to the opticalreconstruction of the object which is formed by illuminating thehologram. The term “replay field” is used to refer to the plane in spacewhere the holographic reconstruction is formed.

The terms “encoding”, “writing” or “addressing” are used to describe theprocess of providing the plurality of pixels of the SLM with a respectplurality of control values which respectively determine the modulationlevel of each pixel. It may be said that the pixels of the SLM areconfigured to “display” a light modulation distribution in response toreceiving the plurality of control values.

The term “light” is used herein in its broadest sense. Embodiments areequally applicable to visible light, infrared light and ultravioletlight, and any combination thereof.

Embodiments describe 1D and 2D holographic reconstructions by way ofexample only. In other embodiments, the holographic reconstruction is a3D holographic reconstruction. That is, in embodiments, eachcomputer-generated hologram forms a 3D holographic reconstruction.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described by way of example only with referenceto the following figures:

FIG. 1 is a schematic showing a reflective SLM producing a holographicreconstruction on a screen;

FIG. 2A illustrates a first iteration of an example Gerchberg-Saxtontype algorithm;

FIG. 2B illustrates the second and subsequent iterations of the exampleGerchberg-Saxton type algorithm;

FIG. 3 is a schematic of a reflective LCOS SLM;

FIG. 3B shows the inherent voltage offset of an operational amplifier inaccordance with the present disclosure; and

FIG. 4 shows an embodiment comprising two single grey-level channels;

FIGS. 5, 6 and 7 show example responses of a light modulating element,such as a pixel comprising liquid crystal, to voltage; and

FIGS. 8, 8B and 8C show three respective examples of the distribution ofgrey levels between the single grey-level channels.

The same reference numbers will be used throughout the drawings to referto the same or like parts.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is not restricted to the embodiments described inthe following but extends to the full scope of the appended claims. Thatis, the present invention may be embodied in different forms and shouldnot be construed as limited to the described embodiments.

Terms of a singular form may include plural forms unless specifiedotherwise.

A structure described as being formed at an upper portion/lower portionof another structure or on/under the other structure should be construedas including a case where the structures contact each other and,moreover, a case where a third structure is disposed there between.

In describing a time relationship—for example, when the temporal orderof events is described as “after”, “subsequent”, “next”, “before” orsuchlike—the present disclosure should be taken to include continuousand non-continuous events unless otherwise specified. For example, thedescription should be taken to include a case which is not continuousunless wording such as “just”, “immediate” or “direct” is used.

Although the terms “first”, “second”, etc. may be used herein todescribe various elements, these elements are not limited by theseterms. These terms are only used to distinguish one element fromanother. For example, a first element could be termed a second element,and, similarly, a second element could be termed a first element,without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled toor combined with each other, and may be variously inter-operated witheach other. Embodiments may be carried out independently from eachother, or may be carried out together in co-dependent relationship.

It has been found that a holographic reconstruction of acceptablequality can be formed from a “hologram” containing only phaseinformation related to the original object. Such a holographic recordingmay be referred to as a phase-only hologram. Embodiments relate tophase-only holography by way of example only. That is, in embodiments,the spatial light modulator applies only a phase-delay distribution toincident light. In embodiments, the phase delay applied by each pixel ismulti-level. That is, each pixel may be set at one of a discrete numberof phase levels. The discrete number of phase levels may be selectedfrom a much larger palette.

In embodiments, the computer-generated hologram is a Fourier transformof the object for reconstruction. In these embodiments, it may be saidthat the hologram is a Fourier domain or frequency domain representationof the object. FIG. 1 shows an embodiment using a reflective SLM todisplay a phase-only Fourier hologram and produce a holographicreconstruction at a replay field.

A light source 110, for example a laser or laser diode, is disposed toilluminate the SLM 140 via a collimating lens 111. The collimating lenscauses a generally planar wavefront of light to be incident on the SLM.The direction of the wavefront is off-normal (e.g. two or three degreesaway from being truly orthogonal to the plane of the transparent layer).In other embodiments, the generally planar wavefront is provided atnormal incidence using a beam splitter, for example. In the exampleshown in FIG. 1, the arrangement is such that light from the lightsource is reflected off a mirrored rear surface of the SLM and interactswith a phase-modulating layer to form an exit wavefront 112. The exitwavefront 112 is applied to optics including a Fourier transform lens120, having its focus at a screen 125.

The Fourier transform lens 120 receives a beam of phase-modulated lightfrom the SLM and performs a frequency-space transformation to produce aholographic reconstruction at the screen 125.

Light is incident across the phase-modulating layer (i.e. the array ofphase modulating elements) of the SLM. Modulated light exiting thephase-modulating layer is distributed across the replay field. Notably,in this type of holography, each pixel of the hologram contributes tothe whole reconstruction. That is, there is not a one-to-one correlationbetween specific points on the replay image and specificphase-modulating elements.

In these embodiments, the position of the holographic reconstruction inspace is determined by the optical power of the Fourier transform lens.In the embodiment shown in FIG. 1, the Fourier transform lens is aphysical lens. That is, the Fourier transform lens is an optical Fouriertransform lens and the Fourier transform is performed optically. Anylens can act as a Fourier transform lens but the performance of the lenswill limit the accuracy of the Fourier transform it performs. Theskilled person understands how to use a lens to perform an opticalFourier transform. However, in other embodiments, the Fourier transformis performed computationally by including lensing data in theholographic data. That is, the hologram includes data representative ofa lens as well as data representing the object. It is known in the fieldof computer-generated hologram how to calculate holographic datarepresentative of a lens. The holographic data representative of a lensmay be referred to as a software lens. A phase-only holographic lens maybe formed, for example, by calculating the phase delay caused by eachpoint of the lens owing to its refractive index and spatially-variantoptical path length. For example, the optical path length at the centreof a convex lens is greater than the optical path length at the edges ofthe lens. An amplitude-only holographic lens may be formed by a Fresnelzone plate. It is also known in the art of computer-generated hologramhow to combine holographic data representative of a lens withholographic data representative of the object so that a Fouriertransform can be performed without the need for a physical Fourier lens.In embodiments, lensing data is combined with the holographic data bysimple vector addition. In embodiments, a physical lens is used inconjunction with a software lens to perform the Fourier transform.Alternatively, in other embodiments, the Fourier transform lens isomitted altogether such that the holographic reconstruction takes placein the far-field. In further embodiments, the hologram may includegrating data—that is, data arranged to perform the function of a gratingsuch as beam steering. Again, it is known in the field ofcomputer-generated hologram how to calculate such holographic data andcombine it with holographic data representative of the object. Forexample, a phase-only holographic grating may be formed by modelling thephase delay caused by each point on the surface of a blazed grating. Anamplitude-only holographic grating may be simply superimposed on anamplitude-only hologram representative of an object to provide angularsteering of an amplitude-only hologram.

A Fourier hologram of a 2D image may be calculated in a number of ways,including using algorithms such as the Gerchberg-Saxton algorithm. TheGerchberg-Saxton algorithm may be used to derive phase information inthe Fourier domain from amplitude information in the spatial domain(such as a 2D image). That is, phase information related to the objectmay be “retrieved” from intensity, or amplitude, only information in thespatial domain. Accordingly, a phase-only Fourier transform of theobject may be calculated.

In embodiments, a computer-generated hologram is calculated fromamplitude information using the Gerchberg-Saxton algorithm or avariation thereof. The Gerchberg Saxton algorithm considers the phaseretrieval problem when intensity cross-sections of a light beam,I_(A)(x, y) and I_(B)(x, y), in the planes A and B respectively, areknown and I_(A)(x, y) and I_(B)(x, y) are related by a single Fouriertransform. With the given intensity cross-sections, an approximation tothe phase distribution in the planes A and B, ψ_(A)(x, y) and ψ_(B)(x,y) respectively, is found. The Gerchberg-Saxton algorithm findssolutions to this problem by following an iterative process.

The Gerchberg-Saxton algorithm iteratively applies spatial and spectralconstraints while repeatedly transferring a data set (amplitude andphase), representative of I_(A)(x, y) and I_(B)(x, y), between thespatial domain and the Fourier (spectral) domain. The spatial andspectral constraints are I_(A)(x, y) and I_(B)(x, y) respectively. Theconstraints in either the spatial or spectral domain are imposed uponthe amplitude of the data set. The corresponding phase information isretrieved through a series of iterations.

In embodiments, the hologram is calculated using an algorithm based onthe Gerchberg-Saxton algorithm such as described in British patent2,498,170 or 2,501,112 which are hereby incorporated in their entiretyby reference.

In accordance with embodiments, an algorithm based on theGerchberg-Saxton algorithm retrieves the phase information ψ[u, v] ofthe Fourier transform of the data set which gives rise to a knownamplitude information T[x, y]. Amplitude information T[x, y] isrepresentative of a target image (e.g. a photograph). The phaseinformation ψ[u, v] is used to produce a holographic representative ofthe target image at an image plane.

Since the magnitude and phase are intrinsically combined in the Fouriertransform, the transformed magnitude (as well as phase) contains usefulinformation about the accuracy of the calculated data set. Thus, thealgorithm may provide feedback on both the amplitude and the phaseinformation.

An example algorithm based on the Gerchberg-Saxton algorithm inaccordance with embodiments of the present disclosure is described inthe following with reference to FIG. 2. The algorithm is iterative andconvergent. The algorithm is arranged to produce a hologram representingan input image. The algorithm may be used to determine an amplitude-onlyhologram, a phase-only hologram or a fully complex hologram. Exampledisclosed herein relate to producing a phase-only hologram by way ofexample only. FIG. 2A illustrates the first iteration of the algorithmand represents the core of the algorithm. FIG. 2B illustrates subsequentiterations of the algorithm.

For the purpose of this description, the amplitude and phase informationare considered separately although they are intrinsically combined toform a composite complex data set. With reference to FIG. 2A, the coreof the algorithm can be considered as having an input comprising firstcomplex data and an output comprising a fourth complex data. Firstcomplex data comprises a first amplitude component 201 and a first phasecomponent 203. Fourth complex data comprises a fourth amplitudecomponent 211 and a fourth phase component 213. In this example, theinput image is two-dimensional. The amplitude and phase information aretherefore functions of the spatial coordinates (x, y) in the farfieldimage and functions of (u, v) for the hologram field. That is, theamplitude and phase at each plane are amplitude and phase distributionsat each plane.

In this first iteration, the first amplitude component 201 is the inputimage 210 of which the hologram is being calculated. In this firstiteration, the first phase component 203 is a random phase component 230merely used as a starting point for the algorithm. Processing block 250performs a Fourier transform of the first complex data to form secondcomplex data having a second amplitude component (not shown) and asecond phase information 205. In this example, the second amplitudecomponent is discarded and replaced by a third amplitude component 207by processing block 252. In other examples, processing block 252performs different functions to produce the third amplitude component207. In this example, the third amplitude component 207 is adistribution representative of the light source. Second phase component205 is quantised by processing block 254 to produce third phasecomponent 209. The third amplitude component 207 and third phasecomponent 209 form third complex data. The third complex data is inputto processing block 256 which performs an inverse Fourier transform.Processing block 256 outputs fourth complex data having the fourthamplitude component 211 and the fourth phase component 213. The fourthcomplex data is used to form the input for the next iteration. That is,the fourth complex data of the nth iteration is used to form the firstcomplex data set of the (n+1)th iteration.

FIG. 2B shows second and subsequent iterations of the algorithm.Processing block 250 receives first complex data having a firstamplitude component 201 derived from the fourth amplitude component 211of the previous iteration and a first phase component 213 correspondingto the fourth phase component of the previous iteration.

In this example, the first amplitude component 201 is derived from thefourth amplitude component 211 of the previous iteration as described inthe following. Processing block 258 subtracts the input image 210 fromthe fourth amplitude component 211 of the previous iteration to formfifth amplitude component 215. Processing block 260 scales the fifthamplitude component 215 by a gain factor α and subtracts it from theinput image 210. This is expressed mathematically by the followingequations:

R _(n+1)[x,y]=F′{exp(i« _(n)[u,v])}

ψ_(n)[u,v]=∠F{η·exp(i∠R _(n)[x,y])}

η=T[x,y]−α(|R _(n)[x,y]|−T[x,y])

Where:

F′ is the inverse Fourier transform;F if the forward Fourier transform;R is the replay field;T is the target image;∠ is the angular information;ψ is the quantized version of the angular information;ε is the new target magnitude, ε≥0; andα is a gain element ˜1.

The gain element a may be fixed or variable. In examples, the gainelement a is determined based on the size and rate of the incomingtarget image data.

Processing blocks 250, 252, 254 and 256 function as described withreference to FIG. 2A. In the final iteration, a phase-only hologram ψ(u,v) representative of the input image 210 is output. It may be said thatthe phase-only hologram ψ(u, v) comprises a phase distribution in thefrequency or Fourier domain.

In other examples, the second amplitude component is not discarded.Instead, the input image 210 is subtracted from the second amplitudecomponent and a multiple of that amplitude component is subtracted fromthe input image 210 to produce the third amplitude component 307. Inother examples, the fourth phase component is not fed back in full andonly a portion proportion to its change over, for example, the last twoiterations is fed back.

In embodiments, there is provided a real-time engine arranged to receiveimage data and calculate holograms in real-time using the algorithm. Inembodiments, the image data is a video comprising a sequence of imageframes. In other embodiments, the holograms are pre-calculated, storedin computer memory and recalled as needed for display on a SLM. That is,in embodiments, there is provided a repository of predeterminedholograms.

However, embodiments relate to Fourier holography and Gerchberg-Saxtontype algorithms by way of example only. The present disclosure isequally applicable to Fresnel holography and holograms calculated byother techniques such as those based on point cloud methods.

The present disclosure may be implemented using any one of a number ofdifferent types of SLM. The SLM may output spatially modulated light inreflection or transmission. In embodiments, the SLM is a liquid crystalon silicon (LCOS) SLM but the present disclosure is not restricted tothis type of SLM.

A LCOS device is capable of displaying large arrays of phase onlyelements in a small aperture. Small elements (typically approximately 10microns or smaller) result in a practical diffraction angle (a fewdegrees) so that the optical system does not require a very long opticalpath. It is easier to adequately illuminate the small aperture (a fewsquare centimetres) of a LCOS SLM than it would be for the aperture of alarger liquid crystal device. LCOS SLMs also have a large apertureratio, there being very little dead space between the pixels (as thecircuitry to drive them is buried under the mirrors). This is animportant issue to lowering the optical noise in the replay field. Usinga silicon backplane has the advantage that the pixels are opticallyflat, which is important for a phase modulating device.

A suitable LCOS SLM is described below, by way of example only, withreference to FIG. 3. An LCOS device is formed using a single crystalsilicon substrate 302. It has a 2D array of square planar aluminiumelectrodes 301, spaced apart by a gap 301 a, arranged on the uppersurface of the substrate. Each of the electrodes 301 can be addressedvia circuitry 302 a buried in the substrate 302. Each of the electrodesforms a respective planar mirror. An alignment layer 303 is disposed onthe array of electrodes, and a liquid crystal layer 304 is disposed onthe alignment layer 303. A second alignment layer 305 is disposed on theliquid crystal layer 304 and a planar transparent layer 306, e.g. ofglass, is disposed on the second alignment layer 305. A singletransparent electrode 307 e.g. of ITO is disposed between thetransparent layer 306 and the second alignment layer 305.

Each of the square electrodes 301 defines, together with the overlyingregion of the transparent electrode 307 and the intervening liquidcrystal material, a controllable phase-modulating element 308, oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels 301 a. By control of the voltageapplied to each electrode 301 with respect to the transparent electrode307, the properties of the liquid crystal material of the respectivephase modulating element may be varied, thereby to provide a variabledelay to light incident thereon. The effect is to provide phase-onlymodulation to the wavefront, i.e. no amplitude effect occurs.

The described LCOS SLM outputs spatially modulated light in reflectionbut the present disclosure is equally applicable to a transmissive LCOSSLM. Reflective LCOS SLMs have the advantage that the signal lines, gatelines and transistors are below the mirrored surface, which results inhigh fill factors (typically greater than 90%) and high resolutions.Another advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer can be half the thickness than would benecessary if a transmissive device were used. This greatly improves theswitching speed of the liquid crystal (a key point for projection ofmoving video images).

The inventor has provided an improved drive solution for a displaydevice such as a LCOS device. Embodiments refer to a LCOS display deviceby way of example only. The present disclosure is applicable to anydisplay device which requires a plurality of analog grey-level voltagesto provide a plurality of grey levels. Embodiments refer to a drivesolution which provides 128 grey levels by way of example only. Thepresent disclosure is applicable to any number of grey levels.

There is provided a method of compensating or adjusting for the voltageoffset variation in the LCOS digital to analog converters. The LCOSbackplane design of the present disclosure comprises one digital toanalog converter per voltage level. Each DAC requires an op-amp, whichtogether form an DAC/op-amp pair (i.e. signal processor). Each op-ampwill experience a random offset voltage, which reduces the precisionwith which the voltage levels can be set. In all cases, the DAC has morebits than grey-levels in the system.

The inventor herein discloses methods for individually compensating forthe voltage offset experienced by the operational amplifier of each ofthe 128 grey-level channels. Embodiments use a switching circuit toenable each of the 128 output voltages to be routed via the switchingcircuit to an output pin where the linearity and performance ismeasured. These measurements may be used in conjunction with the targetgrey level voltage to minimise the voltage error. The voltage accuracyfor each phase level Is particularly important for phase-onlyholographic systems because the voltage error is a phase error whichmanifests itself as random noise in the image.

Notably, the present disclosure relates to a device wherein there isprovided a signal processor comprising a DAC/op-amp pair for eachgrey-level channel. Each DAC/op-amp pair provides only one output.Specifically, each DAC/op-amp pair is uniquely dedicated to providingjust one grey level voltage at any one time, in particular during agiven display time interval (e.g. frame or sub-frame). It may thereforebe understood that each DAC/op-amp pair provides a single grey-levelvoltage, corresponding to one of a plurality of grey levels. The presentdisclosure therefore relates to providing a plurality of singlegrey-level channels using a corresponding plurality of DAC/op-amp pairs.For example, if the drive circuit is required to produce 128 greylevels, there will be 128 DAC/op-amp pairs of 128 correspondinggrey-level channels.

FIG. 3B plots voltage output versus binary input. The response of anideal DAC/op-amp pair is represented by line 370. However, in practice,a DAC/op-amp pair in accordance with the present disclosure has aresponse as represented by line 360 owing to imperfections in themanufacturing process of the op-amp. A voltage offset 350 is shown onthe y-axis. The voltage offset 350 means that each binary input to thegrey-level channel does not give rise to the expected voltage output.Furthermore, each op-amp has a tolerance which manifests itself as atolerance in the voltage output. In some applications this tolerance inthe voltage output is entirely acceptable. However, this tolerance isnot acceptable for some applications disclosed herein.

In some embodiments, the display device comprises a plurality oflight-modulating pixels wherein each light-modulating pixel is arrangedto modulate light by an amount determined by a voltage applied acrossthe active element—for example, liquid crystal—of that pixel.

In some embodiments, the display device comprises a plurality of pixelsoperable at a plurality of grey levels in accordance with a respectiveplurality of analog drive voltages, wherein each pixel isselectively-connectable to the output of any one single grey-levelchannel of the plurality of single grey-level channels. In someembodiments, the pixels comprise liquid crystal. In some embodiments,each pixel is arranged to modulate a parameter of light passing throughthe pixel, wherein each grey level is a modulation level. In someembodiments, the parameter is phase and each modulation level is aphase-delay level. In some embodiments, the pixels are arranged tooperate as individual and independent phase-modulators, optionally,liquid crystal phase-modulators.

The use of liquid crystal as a phase-modulator is well-understood,however, what is less well-known is that the accuracy of phasemodulation is directly responsible for the image quality particularlycontrast. For a phase-only holographic display, for example, it is ofparamount importance to achieve precise voltage control of the liquidcrystal which gives rise to the phase modulation. This voltage precisionrequires a digital to analog converter with a large number of bits andthereby a large number of voltage steps for any given voltage range. Forexample, in some embodiments, the digital signal received at the inputis an 8-bit binary signal. If a particular LCOS backplane design usesone DAC per grey/phase level, then associated with that will be anop-amp circuit to improve the load-driving capability. In someembodiments, each pixel requires a voltage up to 5 V. If 128 evenlyspaced grey levels are required, for example, then the grey levelvoltages should be separated by approximately 40 mV. The offset voltagevariation as part of the op-amp characteristics cannot be guaranteed andcould be in the range+/−100 mV. If less than 100 mV precision isrequired (which it is for 128 grey levels) then it is not possible touse this approach. However, the inventor has addressed this problem inorder that a DAC/op-amp pair may be used for each grey level with therequired voltage precision.

FIG. 4 shows an embodiment comprising a first single grey-level channel410 and a second single grey-level channel 420. FIG. 4 shows two singlegrey-level channels by way of example only. It may be understood thatthe present disclosure extends to any plurality of single grey-levelchannels, such as 128. As described above, each single grey-levelchannel is used to provide a voltage level to drive pixels of a displaydevice at one of a plurality of discrete grey levels. First singlegrey-level channel 410 comprises a first input 412 and first output 418.The first input 412 and first output 418 are connected, in series, by afirst DAC 414 and first op-amp 416. First DAC 414 and first op-amp 416collectively form a first signal processor. Second single grey-levelchannel 420 comprises a second input 422 and second output 428. Thesecond input 422 and second output 428 are connected, in series, by asecond DAC 424 and second op-amp 426. Second DAC 424 and second op-amp426 collectively form a second signal processor. The first output 418and second output 428 are connected to switching circuit 430 which is,in turn, connected to a calibration sub-system or circuit 440 outputtinga feedback parameter 445.

For example, first input 412 may be hexadecimal signal 0100h. First DAC414 converts this digital signal to analog and first op-amp 416 providessuitable driving capability. It is known in the art of signal processinghow to configure a DAC and op-amp for this purpose and no furtherdescription is therefore required here. It is also known that an op-ampcan incorporated others components and/or circuitry in order to alter ortune the voltage offset of the op-amp. For example, it is known that anop-amp might include an input voltage offset. The first output 418provides a voltage arranged to drive a pixel or pixels of the displaydevice at a first grey level.

Likewise, second input 422 may be hexadecimal signal 0101h. DAC 424converts this digital signal to analog and op-amp 426 provides suitableamplification. The second output 428 provides a voltage arranged todrive a pixel or pixels of the display device at a second grey level.

The first output 418 is therefore a first grey level voltage for thedisplay device. The second output 428 is therefore a second grey levelvoltage for the display device. It may be understood how the voltagedifference between the grey level voltages may be chosen for theparticular display device. More specifically, the voltage difference 450between first output 418 and second output 428 may be chosen based onthe working parameters of the display device. In embodiments, 128 greylevels (therefore, 128 single grey-level channels) are provided and thevoltage difference 450 between adjacent grey levels (e.g. grey level0100h and grey level 0101h) may be a few mV. It may be understood howthe full working voltage range of the display device may be divided—e.g.evenly divided—between the grey levels.

Switching circuit 430 is configured to receive each voltage output ofthe single grey-level channels in turn, as part of a calibrationprocess. Calibration sub-system or circuit 440 receives and analyseseach received voltage output from switching circuit 430. In particular,calibration sub-system or circuit 440 may compare a received voltageoutput with a reference voltage for that grey level. For example, thereference voltage may represent a target voltage for that grey level. Alook-up table may be used to determine a feedback parameter for theop-amp of each single grey-level channel based on the correspondingoutput voltage. The feedback parameter directly or indirectly affects ordetermines the offset voltage of the op-amp. In embodiments, thefeedback parameter is a parameter of the op-amp or op-amp circuitry. Itis known in the art how an op-amp may have associated components, suchas resistors, tuning the behaviour of the op-amp. The op-amp and itsassociated components may be consider as forming an op-amp circuit. Inembodiments, the feedback parameter is the value of an electricalcomponent of the op-amp or op-amp circuit. For example, the feedbackparameter may be a voltage, e.g. a voltage for one terminal of theop-amp, or a resistance e.g. the value of a variable resistor formingpart of the op-amp circuit. It may be understood how the switchingcircuit and calibration sub-system or circuit may therefore be used toensure each single grey-level channel provides the correct voltageoutput based on the comparison of the received voltage output from asingle grey-level channel with a reference voltage and using thefeedback parameter. For example, if the voltage output from one singlegrey-level channel falls, a different feedback parameter may beidentified in the look-up table with the effect of adjusting the offsetvoltage of the associated op-amp in order to increase the grey levelvoltage. That is, the feedback parameter is used to fine-tune orcalibrate the offset voltage of the op-amp for each grey-level channel.Since the op-amp of each single grey-level channel may experience adifferent voltage offset from the op-amps of the other grey-levelchannels, as described above with reference to FIG. 3B, the individualcalibration of the op-amp for each grey-level channel compensates forthe random offset voltage experienced by op-amps. It may therefore beunderstood that embodiments provide a method of individuallycompensating for the random offset voltage experienced by each op-amp ofa grey-level channel by self-calibrating a drive circuit and displaydevice pair. For example, this calibration process may be run only onceupon first switch-on, every time the device is switched on and/or atsuitable refresh points and/or run periodically during operation. Eachgrey level of the drive circuit may therefore be individuallycalibrated.

There is therefore provided a system including a display devicecomprising a drive circuit, the drive circuit comprising: a plurality ofsingle grey-level channels, wherein each single grey-level channelcomprises an input, an output and a signal processor connected betweenthe input and output, wherein each processor is arranged to convert adigital signal received at the input into an analog voltage at theoutput, and wherein each signal processor comprises a digital-to-analogconverter, “DAC”, and an operational amplifier, “op-amp”, having avoltage offset; a switching circuit connected to the output of eachsingle grey-level channel, wherein the switching circuit is arranged toswitchably-receive the analog voltage of each single grey-level channelof the plurality of single grey-level channels; a calibration sub-systemconnected to each op-amp, wherein the calibration sub-system is arrangedto receive each analog voltage from the switching circuit andindividually compensate for the voltage offset of each op-amp based onthe received analog voltage for that grey-level channel. The sub-systemmay be a circuit. The sub-system may be internal or external to thebackplane of the display device.

In alternative embodiments, another method of individually compensatingfor the random offset voltage experienced by each op-amp of a grey-levelchannel is used. In particular, the voltage output of each DAC/op-amppair is measured at a plurality of binary inputs (see, for example, V1,V2 . . . V6 in FIG. 3B). These measurements are analysed and storedeither in a look-up table or used to calculate an approximationalgorithm on a per grey level basis. That is, for each single grey-levelchannel, the voltage output is measured at a plurality of binary inputsand the measured values are stored. The stored measurements representthe particular voltage or output response of the signal processorcomprising the DAC/op-amp pair of each single grey-level channel. Thevoltage response for each grey-level channel may be analysed and used toprovide the correct grey level voltage for that grey-level channel.Therefore, when a specific voltage is required, the required binaryinput is looked up (or calculated) and the correct voltage is suppliedto the display device backplane. This calibration process, comprisinganalysing the measurements of voltage output in response to a pluralityof binary inputs to determine the output response across an operatingrange, is required on a per piece basis. That is, this calibrationprocess is carried out on each single grey-level channel. FIG. 3B showsan example DAC/op-amp pair exhibiting a linear response by way ofexample only. It may be understood that each DAC/op-amp pair may exhibitany type of response including a non-linear response, for example. Inembodiments, the calibration sub-system or circuit 440 includes ananalog-to-digital converter (ADC) and a comparator arranged to comparethe output of the ADC with the binary input to the corresponding DAC.These components may be external to the backplane or internal.

It may therefore be understood that, in some embodiments, thecalibration sub-system is arranged to individually determine the outputresponse of each single grey-level channel to a plurality of digitalinputs and, for each single grey-level channel, determine the digitalinput required to achieve each analog drive voltage at the output basedon the individually-determined output response of the single grey-levelchannel. The output response of each single grey-level channel may bedetermined by any means. In some embodiments, the calibration sub-systemis arranged to individually determine the output response of each singlegrey-level channel by measuring the output response of each singlegrey-level channel to a plurality of digital inputs. In someembodiments, the calibration sub-system is arranged to interpolatebetween measured output responses of that single grey-level channel.

FIG. 5 shows a first example response of a liquid crystal to voltage,V_(LC). In some embodiments, the response of the pixels to analogvoltage is measured. In other embodiments, the grey level response ofthe pixels is a performance characteristic of the display device definedby a manufacturer, for example. Many liquid crystals do not respond tovoltages less than 0.7 V. It may also be seen from FIG. 5, for example,that in order to provide equally-spaced grey levels, the requiredcorresponding voltages are not necessarily equally spaced. FIG. 5 showsfour grey levels by way of example only. In some embodiments, 128 greylevels are required but, again, the present disclosure is equallyapplicable to any number of grey levels. By understanding the responseof the liquid crystal and the response of each DAC/op-amp pair, thebinary input required to achieve each required (e.g. evenly spaced) greylevel may be determined for each single grey-level channel.

In some embodiments, the calibration sub-system is arranged to selectthe plurality of grey levels based on the grey level response of thepixels to analog voltage, and determine the plurality of analog drivevoltages required by the pixels to achieve the respective plurality ofgrey levels based on the grey level response of the pixels. The greylevels may have a predetermined distribution (e.g. spacing between greylevels) in a range of grey level values, according to applicationrequirements. In some embodiments, the plurality of grey-levels areevenly-spaced between a lower grey-level and an upper grey-level of eachpixel, optionally, evenly-spaced between a minimum grey-level and amaximum grey-level of each pixel. However, the present disclosure is notlimited in this respect and the grey levels may be unevenly spaced, forexample.

FIG. 6 shows a second example response of a liquid crystal system tovoltage, V_(LC). Again, the required voltage to achieve evenly spacedgrey levels may be determined and correlated with the measured behaviourof each DAC/op-amp pair to provide a look-up table. In some embodiments,the response behaviour of the liquid crystal is dynamically changeable(e.g. can be manipulated during display). This is achieved because, inembodiments, each single grey-level channel is calibrated over a range.In some embodiments, each single grey-level channel is fully calibratedover its full working range.

FIG. 7 shows a third example response of a liquid crystal system tovoltage, V_(LC). This third example response is substantially inverse tothe first example response. For a phase-only image, reversing the liquidcrystal response will vertically flip the image.

In accordance with the above alternative embodiments, a method fordriving a liquid crystal-based display uses a two-stage calibrationprocess. A first stage selects a plurality of discrete grey levels basedon the response of a liquid crystal system. The selected discrete greylevels may be evenly spaced. The grey level response of the liquidcrystal system may be measured or otherwise obtained (e.g. frommanufacturer data). Based on the pixel response, the process determinesthe plurality of analog drive voltage required by the liquid crystalpixels to achieve each grey level of the selected plurality of greylevels. In a second stage, the process individually determines theoutput response of the signal processor comprising a DAC/op-amp pair foreach grey-level channel to a plurality of binary input voltages. Theprocess analyses the determined output response of each grey-levelchannel, and determines the corresponding binary inputs required toprovide the plurality of analog drive voltages determined by the firststage. In particular, the second stage correlates the signal processoroutput response, for each grey-level channel, and the liquid crystalsystem response from the first stage, and then determines, for eachgrey-level channel, the binary input voltages required to provide theplurality of analog drive voltages to achieve each of the plurality ofgrey levels for the liquid crystal system. Thus, each grey-level channelis calibrated across an operating voltage range such as 0 to 5 or 6V.The binary input voltages required for the plurality of grey levels maybe stored in a look-up table for each grey-level channel. Thus, eachgrey-level channel is able to provide an output voltage corresponding toany one of the plurality of discrete grey levels, using thecorresponding binary input from the look-up table, and to producesubstantially the same pixel response as the other grey-level channels.Thus, the method individually compensates for the random offset voltagesexperienced by the op-amps of the multiple grey-level channels. Themethod compensates for variations in the response of differentgrey-levels channels for a plurality of voltage levels, corresponding tothe selected plurality of discrete grey levels (e.g. evenly spaced greylevels). Thus, the method achieves a consistent pixel response to analogoutput voltages provided by different grey-level channels.

The method according to the alternative embodiments may be performed bya switching circuit 430 and a calibration sub-system 440 as shown inFIG. 4. In particular, switching circuit 430 may be configured toreceive the voltage output, in response to each of a plurality of binaryinput voltages, of each single grey-level channel, in turn. Thecalibration sub-system 440 may include any suitable processing systemconfigured to receive and analyse the voltage outputs from the switchingcircuit 430 in accordance with the method. In particular, thecalibration sub-system 430 may determine the signal processor outputresponse for each grey-level channel, and correlate each signalprocessor response and the liquid crystal system response. Thecalibration sub-system 430 may perform measurements to determine theliquid crystal system response, select a plurality of discrete greylevels and determined the corresponding analog drive voltages, or obtainsuch data from elsewhere (e.g. an external system or an internal datastorage). Thus, the calibration sub-system 430 may produce a look-uptable (or equivalent algorithm) that can be used to identify the binaryinput voltage required by each grey-level channel to provide the correctanalog output voltage level to drive pixels of the particular displaydevice at each of the selected plurality of discrete grey levels. Inoperation, the calibration sub-system 430 or other suitable component ofthe driver may use the look-up table (or algorithm) to provide afeedback parameter 445 to each grey-level channel, where the feedbackparameter 445 indicates the required binary input for the grey levelcurrently assigned to the grey-level channel. Thus, the calibrationsub-system 440 is able to individually control each grey-level channelto drive the display device, to compensate for variations as describedherein.

In embodiments, viscosity changes in the liquid crystal are compensatedby dynamically altering the response of the liquid crystal to voltageaccording to temperature to guarantee that the full range of greylevels—e.g. a full 2π phase—is achievable. In other embodiments, theresponse is altered according to the colour of the incident light formodulation.

In some embodiments, the system is arranged to change the grey-levelresponse of the pixels and repeat the individual calibration of thesingle grey-level channels, to compensate for the random offset voltagesexperienced thereby, as described above. In some embodiments, the greylevel response of the pixels is adjusted by changing the lightmodulating element of the pixels (i.e. a change internal to the pixel).

In some embodiments, a grey-level is assigned to each single grey-levelchannel and the digital input required by each single grey-level channelto achieve the respective assigned grey level is established. A look-uptable may be used to record this information. For example, if it isdetermined that grey level 64 is required, the look-up table willidentify which single grey-level channel—e.g. channel 12—has beenassigned grey level 64 and extract the digital input—e.g. 0101h—requiredby channel 12 to achieve grey level 64. The output of channel 12 is thenapplied to the pixel (or even pixels) requiring grey level 64. In someembodiments, the assignment of grey levels to grey-level channels may befixed, so that each single grey-level channel always provides an outputvoltage to drive pixels of a display device at a particular one of aplurality of discrete grey levels. In other embodiments, as describedbelow, the assignment of grey levels to grey-level channels may bedynamically changed between display time intervals. In embodiments inwhich the assignment of grey levels to grey-level channels may bechanged, the assignment of grey levels remains fixed for the duration ofa display time interval, such as a sub-frame or frame.

FIGS. 8, 8B and 8C show example electro-optic responses of the drivecircuit in accordance with some embodiments. More specifically, FIGS. 8,8B and 8C shows how the plurality of grey-level channels may be used toprovide a respective plurality of grey levels—such as phase levels. FIG.8A shows a first configuration in which the first channel provides thelowest grey level and each successive channel provides the next greylevel. The last channel therefore provides the highest grey level. FIGS.8B and 8C show respective second and third configurations in whichsuccessive channels generally provide the next grey level but the lowestgrey-level channel is not at either extremity. Accordingly, there is adiscontinuity in the grey level distribution. It may be said that awrap-around distribution of grey levels between the channels isprovided.

In some embodiments, the method comprises changing the distribution ofgrey levels between the channels. It may be said that the methodcomprises moving, changing or varying—including dynamically varying—thediscontinuity in the wrap-around distribution. In some embodiments, thedistribution of grey levels between the channels is changed duringdisplay—for example, between frames of a sequence of frames. In otherembodiments, each frame is composed of or comprises identical orsubstantially identical sub-frames—e.g. because the display devicerequires a refresh—and the distribution is changed between sub-frames.That is, in some embodiments, the distribution is dynamically changedduring display. It may be said that the method comprises changing theassignment of grey-levels between the single grey-level channels duringdisplay. The inventor has identified that the randomness introduced bydynamically changing the grey-level distribution between the singlegrey-level channels provides a despeckling effect in the image. Thisrandomness is sufficient to at least partially compensate for therandomness of laser speckle. There is therefore provided a computationalmethod of reducing speckle. An improved image is therefore provided. Insome embodiments, the pixels are phase modulating pixels and the greylevels are phase values in the range 0 to 2π, for example. In someembodiments, the phase-delay distribution between the channels isdynamically changed (e.g. shifted back and forth by π/2) in order toreduce the noise in the image caused by speckle.

In some embodiments, a look-up table is provided which dictates whichchannel is used to provide each grey-level and what digital input eachchannel requires to achieve the assigned grey-level. The look-up tablemay be generated based on the analysis from the calibration process andused during operation of the drive method.

Accordingly, methods are provided for driving a display device that usesa plurality of grey-level channels. Each grey-level channel comprises asignal processor that provides an output voltage at a voltage level fordriving the pixels of the display at one of a plurality of discrete greylevels. The method compensates for random variations in the differentgrey-level channels, in particular random variations in the voltageoffset of an op-amp in a signal processor of a DAC/op-amp pair, by meansof a calibration process. Embodiments analyse one or more analog voltageoutputs of each grey-level channel. Calibration may be performed basedon the analysis. In some embodiments, the analysis compares an analogoutput voltage to a reference voltage (e.g. target voltage) for agrey-level channel. In these embodiments, the method individuallycalibrates each grey-level channel using a feedback parameter thatchanges a value of an electrical component thereof. Thus, eachcalibrated grey-level channel provides a target voltage output for thecorresponding grey level. In other embodiments, the analysis determinesthe output response of each grey-level channel to a plurality of digitalinputs across an operating range thereof. Based on the output response,the analysis further determines a digital input required to achieve atarget analog output voltage corresponding to one or more of a pluralityof grey levels. The plurality of grey levels may be selected based onthe pixel (e.g. liquid crystal) response, and the corresponding analogoutput voltages determined therefrom. Thus, embodiments correlate theresponse of each grey-level channel to the grey level pixel response.Since each grey-level channel is calibrated across a range of voltages,it may be assigned to any one of the plurality of grey levels at any onetime. Thus, the assignment of grey levels may be dynamically changed tointroduce a randomness which at least partially compensates for laserspeckle. Furthermore, when the assignment of grey levels is dynamicallychanged, the different grey-level channels provide voltages that producea consistent pixel response for the same grey level.

It may be understood that the present disclosure provides techniquesthat compensate for the voltage offset of an op-amp of a signalprocessor of a single grey-level channel, to mitigate the disadvantagesassociated with the variation of the voltage offset experienced bydifferent op-amps due to random variations. Accordingly, the embodimentsdescribed herein may not eliminate a voltage offset associated with eachop-amp, but rather may compensate for the effects of the variation inthe voltage offsets amongst the signal processors, and thus the multiplegrey-level channels. In particular, embodiments aim to operate eachsingle grey-level channel to provide an analog output voltage that is asclose as possible to the desired or target voltage for the grey levelassigned to that channel. The term “compensate/compensating for thevoltage offset” should be understood in light of the above, and itsmeaning is not limited to eliminating or reducing a voltage offsetexperienced by an op-amp of a signal processor.

In embodiments, the spatially light modulator is a phase-only spatiallight modulator. These embodiments are advantageous because no opticalenergy is lost by modulating amplitude. Accordingly, an efficientholographic projection system is provided. However, the presentdisclosure may be equally implemented on an amplitude-only spatial lightmodulator or an amplitude and phase modulator. It may be understood thatthe hologram will be correspondingly phase-only, amplitude-only orfully-complex.

In embodiments, the light source is a laser. In embodiments, thedetector is a photodetector. In embodiments, the screen is a diffuser.The holographic projection system of the present disclosure may be usedto provide an improved head-up display or head-mounted display. Inembodiments, there is provided a vehicle comprising the holographicprojection system.

Although groups of embodiments have been largely disclosed separately,any feature of any embodiment or group of embodiments may be combinedwith any other feature or combination of features of any embodiment orgroup of embodiments. That is, all possible combinations andpermutations of features disclosed in the present disclosure areenvisaged.

The quality of the holographic reconstruction may be affect by theso-called zero order problem which is a consequence of the diffractivenature of using a pixelated spatial light modulator. Such zero-orderlight can be regarded as “noise” and includes for example specularlyreflected light, and other unwanted light from the SLM.

In the example of Fourier holography, this “noise” is focussed at thefocal point of the Fourier lens leading to a bright spot, known as the“DC spot”, at the centre of the holographic reconstruction. The zeroorder light may be simply blocked out however this would mean replacingthe bright spot with a dark spot. Embodiments include an angularlyselective filter to remove only the collimated rays of the zero order.Embodiments also include the method of managing the zero-order describedin European patent 2,030,072 which is hereby incorporated in itsentirety by reference.

Whilst embodiments described herein include displaying one hologram perframe on the spatial light modulator, the present disclosure is by nomeans limited in this respect and more than one hologram may bedisplayed on the SLM at any one time. For example, embodiments implementthe technique of “tiling”, in which the surface area of the SLM isfurther divided up into a number of tiles, each of which is set in aphase distribution similar or identical to that of the original tile.Each tile is therefore of a smaller surface area than if the wholeallocated area of the SLM were used as one large phase pattern. Thesmaller the number of frequency component in the tile, and respectivelythe larger the number of tiles, the further apart the reconstructedpixels are separated when the image is produced. The image is createdwithin the zeroth diffraction order, and it is preferred that the firstand subsequent orders are displaced far enough so as not to overlap withthe image and may be blocked by way of a spatial filter.

As mentioned above, the holographic reconstruction produced by thismethod (whether with tiling or without) comprises spots that form imagepixels. The higher the number of tiles used, the smaller these spotsbecome. If one takes the example of a Fourier transform of an infinitesine wave, a single frequency is produced. This is the optimum output.In practice, if just one tile is used, this corresponds to an input of asingle cycle of a sine wave, with a zero values extending in thepositive and negative directions from the end nodes of the sine wave toinfinity. Instead of a single frequency being produced from its Fouriertransform, the principle frequency component is produced with a seriesof adjacent frequency components on either side of it. The use of tilingreduces the magnitude of these adjacent frequency components and as adirect result of this, less interference (constructive or destructive)occurs between adjacent image pixels, thereby improving the imagequality. Preferably, each tile is a whole tile, although embodiments usefractions of a tile.

In examples disclosed herein, three different colour light sources andthree corresponding SLMs are used to provide composite colour. Theseexamples may be referred to as spatially-separated colour, “SSC”. In avariation encompassed by the present disclosure, the different hologramsfor each colour are displayed on different area of the same SLM and thencombining to form the composite colour image. However, the skilledperson will understand that at least some of the devices and methods ofthe present disclosure are equally applicable to other methods ofproviding composite colour holographic images.

One of these methods is known as Frame Sequential Colour, “FSC”. In anexample FSC system, three lasers are used (red, green and blue) and eachlaser is fired in succession at a single SLM to produce each frame ofthe video. The colours are cycled (red, green, blue, red, green, blue,etc.) at a fast enough rate such that a human viewer sees apolychromatic image from a combination of the three lasers. Eachhologram is therefore colour specific. For example, in a video at 25frames per second, the first frame would be produced by firing the redlaser for 1/75th of a second, then the green laser would be fired for1/75th of a second, and finally the blue laser would be fired for 1/75thof a second. The next frame is then produced, starting with the redlaser, and so on.

An advantage of FSC method is that the whole SLM is used for eachcolour. This means that the quality of the three colour images producedwill not be compromised because all pixels on the SLM are used for eachof the colour images. However, a disadvantage of the FSC method is thatthe overall image produced will not be as bright as a correspondingimage produced by the SSC method by a factor of about 3, because eachlaser is only used for a third of the time. This drawback couldpotentially be addressed by overdriving the lasers, or by using morepowerful lasers, but this would require more power to be used, wouldinvolve higher costs and would make the system less compact.

An advantage of the SSC method is that the image is brighter due to allthree lasers being fired at the same time. However, if due to spacelimitations it is required to use only one SLM, the surface area of theSLM can be divided into three parts, acting in effect as three separateSLMs. The drawback of this is that the quality of each single-colourimage is decreased, due to the decrease of SLM surface area availablefor each monochromatic image. The quality of the polychromatic image istherefore decreased accordingly. The decrease of SLM surface areaavailable means that fewer pixels on the SLM can be used, thus reducingthe quality of the image. The quality of the image is reduced becauseits resolution is reduced.

Examples describe illuminating the SLM with visible light but theskilled person will understand that the light sources and SLM mayequally be used to direct infrared or ultraviolet light, for example, asdisclosed herein. For example, the skilled person will be aware oftechniques for converting infrared and ultraviolet light into visiblelight for the purpose of providing the information to a user. Forexample, the present disclosure extends to using phosphors and/orquantum dot technology.

The methods and processes described herein may be embodied on acomputer-readable medium. The term “computer-readable medium” includes amedium arranged to store data temporarily or permanently such asrandom-access memory (RAM), read-only memory (ROM), buffer memory, flashmemory, and cache memory. The term “computer-readable medium” shall alsobe taken to include any medium, or combination of multiple media, thatis capable of storing instructions for execution by a machine such thatthe instructions, when executed by one or more processors, cause themachine to perform any one or more of the methodologies describedherein, in whole or in part.

The term “computer-readable medium” also encompasses cloud-based storagesystems. The term “computer-readable medium” includes, but is notlimited to, one or more tangible and non-transitory data repositories(e.g., data volumes) in the example form of a solid-state memory chip,an optical disc, a magnetic disc, or any suitable combination thereof.In some example embodiments, the instructions for execution may becommunicated by a carrier medium. Examples of such a carrier mediuminclude a transient medium (e.g., a propagating signal that communicatesinstructions).

Aspects of the present disclosure are set out below.

A display device comprising a drive circuit and corresponding method isprovided. The drive circuit comprises a plurality of single grey-levelchannels, wherein each single grey-level channel comprises an input, anoutput and a signal processor connected between the input and output.Each signal processor is arranged to convert a digital signal receivedat the input into an analog voltage at the output, and comprises adigital-to-analog converter, “DAC”, and an operational amplifier,“op-amp”, having a voltage offset. A switching circuit is connected tothe output of each single grey-level channel, wherein the switchingcircuit is arranged to switchably-receive the analog voltage of eachsingle grey-level channel of the plurality of single grey-levelchannels. A calibration sub-system of circuit receives each analogvoltage from the switching circuit and individually calibrates thevoltage offset of each op-amp based on the received analog voltage forthat grey-level channel. The calibration sub-system may be arranged toanalyse the analog voltages, for example by comparing each analogvoltage of a single grey-level channel to a corresponding referencevoltage. Calibration may be performed based on the analysis.

In implementations, the calibration sub-system or circuit may bearranged to individually calibrate the voltage offset of each op-amp byindividually selecting a feedback parameter for each op-amp. A look-uptable may be provided, which identifies the feedback parameter for eachop-amp based on the corresponding analog voltage received by theswitching circuit. The feedback parameter may be the value of anelectrical component of the op-amp, or the value of an electricalcomponent in the associated op-amp circuit.

Each single grey-level channel provides a single grey level voltage. Inexamples, there are 128 single grey-level channels respectivelyproviding 128 grey level voltages.

The display device may be a spatial light modulator, for example aLiquid Crystal on Silicon, “LCOS”, spatial light modulator.

There is provided a method for calibrating a LCOS device havingbackplane comprising one DAC per grey level, wherein each DAC comprisesan op-amp. The method comprises routing each of the 128 output voltagesto an output pin using a switching circuit; measuring each outputvoltage; and using this measurement in conjunction with a target greylevel voltage to minimise the voltage error in the op-amps.

There is provided a method for a display device comprising a drivecircuit. The drive circuit comprises a plurality of single grey-levelchannels, wherein each single grey-level channel comprises an input, anoutput and a signal processor connected between the input and output.Each signal processor comprises a digital-to-analog converter, “DAC”,and an operational amplifier, “op-amp”, having a voltage offset. Adigital signal received at the input is converted into an analog voltageat the output using each respective signal processor. An analog voltageof each single grey-level channel of the plurality of single grey-levelchannels is switchably-received, for example using a switching circuitconnected to the output of each single grey-level channel. Each analogvoltage is received, for example by a calibration sub-system from theswitching circuit. The method individually compensates for the voltageoffset of each op-amp based on the received analog voltage for thatgrey-level channel. For example, the received analog voltage for agrey-level channel is analysed. Based on the analysis, the drive circuitis operated to individually compensate for the voltage offset of eachop-amp based on the received analog voltage for that grey-level channel.

In implementations, the display device comprises a plurality of pixelsoperable at a plurality of grey levels in accordance with a respectiveplurality of analog drive voltages. Each pixel isselectively-connectable to the output of any one single grey-levelchannel of the plurality of single grey-level channels. Each pixel maybe arranged to modulate a parameter of light passing through the pixel,wherein each grey level is a modulation level. For example, theparameter is phase and each modulation level is a phase-delay level.

The digital signal received at the input may be an 8-bit binary signal.

In implementations, the output response of each single grey-levelchannel to a plurality of digital inputs is further determined. For eachsingle grey-level channel, the digital input required to achieve eachanalog drive voltage at the output is determined, based on the outputresponse of the single grey-level channel.

Individually determining the output response of each single grey-levelchannel may comprise measuring the output response of each singlegrey-level channel to a plurality of digital inputs. In implementations,interpolating between the measured output responses of that singlegrey-level channel may be used to determine the output response.

In implementations, the method comprises selecting the plurality of greylevels based on the grey level response of the pixels to analog voltage;and determining the plurality of analog drive voltages required by thepixels to achieve the respective plurality of grey levels based on thegrey level response of the pixels.

The plurality of grey-levels may be evenly-spaced between a lowergrey-level and an upper grey-level of each pixel, optionally,evenly-spaced between a minimum grey-level and a maximum grey-level ofeach pixel. However, any desired distribution (e.g. spacing) of the greylevels for the pixels is possible of a particular display deviceaccording to application requirements.

In implementations, a grey-level is assigned to each single grey-levelchannel and the method establishes the digital input required by eachsingle grey-level channel to output the required analog voltage for therespectively-assigned grey-level. The assignment of grey-levels may bechanged between the single grey-level channels during display, forexample between display time intervals such as frames or sub-frames.

In implementations, the grey level response of the pixels to analogvoltage may be changed. In this case, the method is repeated. Thus, theoutput response of each grey-level channel is correlated with the newgrey level pixel response. Changing the grey level response of eachpixel may comprise making a change internal to the pixel or a changeexternal to the pixel.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thescope of the appended claims. The present disclosure covers allmodifications and variations within the scope of the appended claims andtheir equivalents.

1-25. (canceled)
 26. A drive method for a display device comprising adrive circuit, the display device comprising a plurality of pixelsoperable at a plurality of grey levels in accordance with a respectiveplurality of analog drive voltages, the drive circuit comprising aplurality of single grey-level channels, wherein each single grey-levelchannel comprises an input, an output and a signal processor connectedbetween the input and output, wherein each signal processor comprises adigital-to-analog converter, “DAC”, and an operational amplifier,“op-amp”, having a voltage offset, the method comprising: converting adigital signal received at the input into an analog voltage at theoutput using each respective signal processor; switchably-receiving theanalog voltage of each single grey-level channel of the plurality ofsingle grey-level channels using a switching circuit connected to theoutput of each single grey-level channel; receiving each analog voltagefrom the switching circuit; and for each single grey-level channel,performing a calibration process to individually compensate for thevoltage offset of each op-amp based on the received analog voltage forthat grey level channel, wherein the calibration process comprises:individually determining the output response of each single grey-levelchannel to a plurality of digital inputs; and for each single grey-levelchannel, determining the digital input required to achieve each analogdrive voltage at the output based on the individually-determined outputresponse of the single grey-level channel, the method furthercomprising: selecting the plurality of grey levels based on the greylevel response of the pixels to analog voltage; and determining theplurality of analog drive voltages required by the pixels to achieve therespective plurality of grey levels based on the grey level response ofthe pixels.
 27. A method as claimed in claim 26 further comprisingselectively-connecting each pixel to the output of any one singlegrey-level channel of the plurality of single grey-level channels.
 28. Amethod as claimed in claim 26 wherein each pixel is arranged to modulatea parameter of light passing through the pixel, wherein each grey levelis a modulation level.
 29. A method as claimed in claim 28 wherein theparameter is phase and each modulation level is a phase-delay level. 30.A method as claimed in claim 26 wherein the digital signal received atthe input is an 8-bit binary signal.
 31. A method as claimed in claim 26wherein individually determining the output response of each singlegrey-level channel comprises measuring the output response of eachsingle-grey level channel to a plurality of digital inputs.
 32. A methodas claimed in claim 31 wherein individually determining the outputresponse of each single grey-level channel further comprises, for eachsingle grey-level channel, interpolating between the measured outputresponses of that single grey-level channel.
 33. A method as claimed inclaim 32 wherein the plurality of grey-levels are evenly-spaced betweena lower grey-level and an upper grey-level of each pixel, optionally,evenly-spaced between a minimum grey-level and a maximum grey-level ofeach pixel.
 34. A method as claimed in claim 26 further comprisingassigning a grey-level to each single grey-level channel andestablishing the digital input required by each single grey-levelchannel to output the required analog voltage for therespectively-assigned grey-level.
 35. A method as claimed in claim 34further comprising changing the assignment of grey-levels between thesingle grey-level channels during display.
 36. A method as claimed inclaim 26 further comprising changing the grey level response of thepixels to analog voltage and repeating the steps of: individuallydetermining the output response of each single grey-level channel to aplurality of digital inputs; and for each single grey-level channel,determining the digital input required to achieve each analog drivevoltage at the output based on the individually-determined outputresponse of the single grey-level channel.
 37. A method as claimed inclaim 36 wherein changing the grey level response of each pixelcomprises making a change internal to the pixel or a change external tothe pixel.
 38. A method as claimed in claim 26 wherein there are 128single grey-level channels respectively providing 128 grey levelvoltages.
 39. A method as claimed in claim 26 wherein the display deviceis spatial light modulator.
 40. A method as claimed in claim 26 whereinthe display device is a Liquid Crystal on Silicon, “LCOS”, spatial lightmodulator.
 41. A system including a display device comprising a drivecircuit, the drive circuit comprising: a plurality of single grey-levelchannels, wherein each single grey-level channel comprises an input, anoutput and a signal processor connected between the input and output,wherein each processor is arranged to convert a digital signal receivedat the input into an analog voltage at the output, and wherein eachsignal processor comprises a digital-to-analog converter, “DAC”, and anoperational amplifier, “op-amp”, having a voltage offset; and aswitching circuit connected to the output of each single grey-levelchannel, wherein the switching circuit is arranged to switchably-receivethe analog voltage of each single grey-level channel of the plurality ofsingle grey-level channels, wherein the system further comprises: acalibration sub-system connected to each op-amp, wherein the calibrationsub-system is arranged to receive each analog voltage from the switchingcircuit and, for each single grey-level channel, perform a calibrationprocess to individually compensate for the voltage offset of each op-ampbased on the received analog voltage for that grey level channel,wherein the calibration process comprises: individually determining theoutput response of each single grey-level channel to a plurality ofdigital inputs; and for each single grey-level channel, determining thedigital input required to achieve each analog drive voltage at theoutput based on the individually-determined output response of thesingle grey-level channel, wherein the calibration sub-system is furtherarranged to select the plurality of grey levels based on the grey levelresponse of the pixels to analog voltage; and determine the plurality ofanalog drive voltages required by the pixels to achieve the respectiveplurality of grey levels based on the grey level response of the pixels.42. A system as claimed in claim 41 wherein the calibration sub-systemis arranged to perform a drive method comprising: converting a digitalsignal received at the input into an analog voltage at the output usingeach respective signal processor; switchably-receiving the analogvoltage of each single grey-level channel of the plurality of singlegrey-level channels using a switching circuit connected to the output ofeach single grey-level channel; receiving each analog voltage from theswitching circuit; and for each single grey-level channel, performing acalibration process to individually compensate for the voltage offset ofeach op-amp based on the received analog voltage for that grey levelchannel, wherein the calibration process comprises: individuallydetermining the output response of each single grey-level channel to aplurality of digital inputs; and for each single grey-level channel,determining the digital input required to achieve each analog drivevoltage at the output based on the individually-determined outputresponse of the single grey-level channel, the method furthercomprising: selecting the plurality of grey levels based on the greylevel response of the pixels to analog voltage; and determining theplurality of analog drive voltages required by the pixels to achieve therespective plurality of grey levels based on the grey level response ofthe pixels; and selectively-connecting each pixel to the output of anyone single grey-level channel of the plurality of single grey-levelchannels.